Find missing files from subdirectories given the file names have sequential number

Question

I have a directory structure like bellow.

.
├── Course 1
│   ├── 1 - Introduction
│   │   ├── 01-First Video.mp4
│   │   ├── 02-Second Video.mp4
│   │   ├── 03-Third Video.mp4
│   ├── 2 - Chapter 1
│   │   ├── 05-Nth Video.mp4
│   │   ├── 06-Nth1 Video.mp4
│   │   └── 07-Nth2 Video.mp4
│   ├── 3 - Chapter 2
│   │   ├── 08-Nth Video.mp4
│   │   ├── 09-Nth1 Video.mp4
│   │   └── 10-Nth2 Video.mp4
│   ├── 4 - Analysis
│   │   ├── 11-Nth Video.mp4
│   │   ├── 12-Nth1 Video.mp4
│   │   └── 13-Nth2 Video.mp4
│   ├── 5 - Conclusion
│   │   ├── 14-Nth Video.mp4
│   │   ├── 15-Nth1 Video.mp4
│   │   └── 16-Nth2 Video.mp4
├── Course 2
│   ├── 1 - Introduction
│   │   ├── 01-First Video.mp4
│   │   ├── 02-Second Video.mp4
│   │   ├── 03-Third Video.mp4
│   ├── 2 - Chapter 1
│   │   ├── 05-Nth Video.mp4
│   │   ├── 06-Nth1 Video.mp4
│   │   └── 07-Nth2 Video.mp4
│   ├── 4 - Analysis
│   │   ├── 11-Nth Video.mp4
│   │   ├── 12-Nth1 Video.mp4
│   │   └── 13-Nth2 Video.mp4
│   ├── 5 - Conclusion
│   │   ├── 14-Nth Video.mp4
│   │   ├── 15-Nth1 Video.mp4
│   │   └── 16-Nth2 Video.mp4
├── Course 3
│   ├── 1 - Introduction
│   │   ├── 02-First Video.mp4
│   │   ├── 03-Third Video.mp4
│   ├── 2 - Chapter 1
│   │   ├── 05-Nth Video.mp4
│   │   └── 07-Nth2 Video.mp4
│   ├── 4 - Analysis
│   │   ├── 11-Nth Video.mp4
│   │   ├── 12-Nth1 Video.mp4
│   ├── 5 - Conclusion
│   │   ├── 14-Nth Video.mp4
│   │   ├── 15-Nth1 Video.mp4
│   │   └── 16-Nth2 Video.mp4

I want to find if any files are missing in the directories.

The code that i am using to solve this problem is:

#!/usr/bin/env bash

previousname="Placeholder"
nextfilenumber=1
find . -type f -name "*.mp4" | sort --version-sort | while read -r line ; do
    coursename=$(echo $line | cut -d/ -f2)
    if [[ "$previousname" != "$coursename" ]]; then
        nextfilenumber=2
        previousname=$coursename
    else
        filename=${line##*/}
        # https://stackoverflow.com/a/24777667/1772898
        filenumber=$((10#${filename%%-*}))
        nextfilenumber="$((nextfilenumber+1))"
        temp="$((filenumber+1))"
        if [[ $temp -ne $nextfilenumber ]]; then
            echo "File(s) Before \"$coursename\"\'s \"$filename\""
            nextfilenumber="$((nextfilenumber+1))"
        fi
    fi
done

How can I make this code better?

Answer 1

Improve usability

For the example input, the script produces the following output for Course 2:

File(s) Before "Course 2"\'s "05-Nth Video.mp4"
File(s) Before "Course 2"\'s "11-Nth Video.mp4"
File(s) Before "Course 2"\'s "12-Nth1 Video.mp4"
File(s) Before "Course 2"\'s "13-Nth2 Video.mp4"

The first line I understand: a file with number 04 is missing, before seeing 05.

The rest of the lines look a bit strange: I understand the objection about the file with number 11, since files with number 08, 09 and 10 are missing before that.

I think it would be more user friendly to print something like:

Expected a file with number 4, got: Course 2/2 - Chapter 1/05-Nth Video.mp4
Expected a file with number 8, got: Course 2/4 - Analysis/11-Nth Video.mp4

---

As another example, consider the output for Course 3:

File(s) Before "Course 3"\'s "03-Third Video.mp4"
File(s) Before "Course 3"\'s "05-Nth Video.mp4"
File(s) Before "Course 3"\'s "07-Nth2 Video.mp4"
File(s) Before "Course 3"\'s "11-Nth Video.mp4"
File(s) Before "Course 3"\'s "12-Nth1 Video.mp4"
File(s) Before "Course 3"\'s "14-Nth Video.mp4"
File(s) Before "Course 3"\'s "15-Nth1 Video.mp4"

Similar to the previous example, this looks a bit strange because a file with number 1 is missing, a file with number 2 is present, but the message mentions a file with number 3.

---

Also note the message prints \' instead of simply '.

Simplify the number comparison logic

Instead of this:

nextfilenumber="$((nextfilenumber+1))"
temp="$((filenumber+1))"
if [[ $temp -ne $nextfilenumber ]]; then
    echo "File(s) Before \"$coursename\"\'s \"$filename\""
    nextfilenumber="$((nextfilenumber+1))"
fi

This is shorter and equivalent:

if [[ $filenumber -ne $nextfilenumber ]]; then
    echo "File(s) Before \"$coursename\"\'s \"$filename\""
    nextfilenumber="$((nextfilenumber+1))"
fi
nextfilenumber="$((nextfilenumber+1))"

The original unnecessarily computes the next file number in the poorly named temp variable.

Remove redundant quotes

The double-quotes in expressions like this are unnecessary:

nextfilenumber="$((nextfilenumber+1))"

They are harmless, so it's ok to keep them, but then it's better to apply it consistently everywhere (example where not used: filenumber=$((10#${filename%%-*}))).

Beware of running commands in a loop

The script runs cut in a loop to extract a path component. Running commands in a loop can make scripts very slow. In this example you can achieve the same result using shell builtins:

IFS=/ read _ coursename _ <<< "$line"

Improve variable names

Renaming previousname to previous_coursename would make the connection with coursename more clear.

Alternative implementation

Addressing the above issues and putting it together:

#!/usr/bin/env bash

previous_course_name=
expected_file_number=1
find . -type f -name "*.mp4" | sort --version-sort | while read -r line ; do
    IFS=/ read _ current_course_name _ <<< "$line"
    if [[ "$previous_course_name" != "$current_course_name" ]]; then
        expected_file_number=1
        previous_course_name=$current_course_name
    fi

    filename=${line##*/}
    # https://stackoverflow.com/a/24777667/1772898
    file_number=$((10#${filename%%-*}))
    if [[ $file_number != $expected_file_number ]]; then
        echo "Expected a file with number $expected_file_number; got: $line"
    fi
    ((expected_file_number = file_number + 1))
done

This produces output:

Expected a file with number 4; got: ./Course 1/2 - Chapter 1/05-Nth Video.mp4
Expected a file with number 4; got: ./Course 2/2 - Chapter 1/05-Nth Video.mp4
Expected a file with number 8; got: ./Course 2/4 - Analysis/11-Nth Video.mp4
Expected a file with number 1; got: ./Course 3/1 - Introduction/02-First Video.mp4
Expected a file with number 4; got: ./Course 3/2 - Chapter 1/05-Nth Video.mp4
Expected a file with number 6; got: ./Course 3/2 - Chapter 1/07-Nth2 Video.mp4
Expected a file with number 8; got: ./Course 3/4 - Analysis/11-Nth Video.mp4
Expected a file with number 13; got: ./Course 3/5 - Conclusion/14-Nth Video.mp4